In accordance with a known technique, short laser impulses illuminating an aircraft during radar surveillance are detected by providing a photodetector which produces electrical impulses which are amplified and fed to the input circuit of an integrator. These impulses consist of signal impulses, one being produced for each burst of laser light detected by the photodetector, together with noise impulses due to ambient light such as sunlight and sunlight reflection from, for example, helicopter rotor blades. Should an integrator integrate noise and signal impulses during a time interval of for example, 0.1 seconds, a ramp voltage is produced having a final amplitude at the end of the interval which is proportional to the integral of the noise and signal impulses of one polarity. Upon the commencement of the receipt of laser signal impulses, the final integrator ramp voltage will increase above a given threshold voltage to provide for the detection of the laser illumination. By applying such ramp voltage to the input circuit of comparator, a comparator output signal may be produced indicating the fact that signal (laser) impulses are present, since the final ramp voltage will exceed a reference threshold voltage.
However, the photodetector output may change, typically 4-1, between shade and full sunlight, which may cause the integrator to saturate, which in turn causes the comparator output to be unreliable as a high noise level may be mistaken for signal impulses due to laser illumination.
It is an object of the present invention to provide detection circuitry having improved sensitivity and reliability relative to the technique explained above.
It is a further object of the invention to provide a sharply reduced false alarm rate and to permit smaller and less expensive photodiodes to be used in detecting certain high pulse repetition frequency laser threats, regardless of the presence of a widely varying noise background.